The present invention relates to the field of exhaust gas cleaning of internal combustion engines. Specifically the invention provides an improved catalyst exhibiting superior performance with respect to poisoning and thermal damage. The invention also provides a process for manufacturing this catalyst.
In the following the invention will be explained with reference to three-way catalysts for cleaning the exhaust gases of stoichiometrically operated gasoline engines. But it should be kept in mind that the underlying principle of the invention is equally well applicable for so-called lean burn and diesel engines.
Three-way catalysts are primarily used to convert the pollutants carbon monoxide (CO), hydrocarbons (HC) and nitrogen oxides (NOx) contained in the exhaust gas of internal combustion engines into harmless substances. Known three-way catalysts with good activity and durability utilize one or more catalytic components from the platinum group metals such as platinum, palladium, rhodium and iridium deposited on a high surface area, refractory oxide support, e.g., a high surface area alumina. The support is usually carried in the form of a thin layer or coating on a suitable carrier or substrate such as a monolithic carrier comprising a refractory ceramic or metal honeycomb structure.
The ever increasing demand for improved catalyst activity and life has led to complex catalyst designs comprising multiple catalyst layers on the carrier structures, each of the layers containing selected support materials and catalytic components as well as so called promoters, stabilizers and oxygen storage compounds.
U.S. Pat. No. 5,010,051 describes a catalyst composition having an upstream stage and a down-stream stage, the upstream stage containing a catalytic material which is different from the catalytic material contained on the downstream stage and is characterized by having a low ignition temperature, e.g., 350° C. to less than 400° C., for the substantially simultaneous conversion of HC, CO and NOx pollutants contained in, e.g., the exhaust of an automobile engine operating at a substantially stoichiometric air-to-fuel weight ratio. The downstream catalytic material is characterized by having a higher conversion efficiency for substantially simultaneous conversion of HC, CO and NOx than the upstream catalytic material at elevated operation temperatures which may be, for example, from about 400 to 800° C.
The U.S. Pat. No. 6,087,298 discloses an exhaust gas treatment catalytic article having an upstream catalytic zone and at least one downstream catalytic zone. The upstream catalytic zone has an upstream composition which has a first upstream support, and at least one first upstream palladium component. The upstream zone can have one or more layers. The downstream catalytic zone has a first downstream layer which has a first downstream support and a first downstream precious metal component. A second downstream layer has a second downstream support and a second downstream precious metal component.
Exhaust gas cleaning catalysts are subjected to high temperatures and exhaust gas components such as sulfur, lead and phosphorous which tend to poison the catalyst especially at the upstream end of the catalyst. Thermal damage is due to the fact that in order to have a light off of the catalysts as early as possible they are placed nearer and nearer to the engine outlet. Thus, during normal operation the upstream end of the catalysts experiences very high temperatures. This results in thermal damage to the catalyst gradually proceeding from the upstream end to the downstream end.
Based on the foregoing there is a need in the art for improving thermal resistance and poisoning resistance of exhaust gas cleaning catalysts especially at the upstream end of these catalysts. It is therefore an object of the present invention to provide such a catalyst and a process for its manufacture.